The present invention relates to a scanning optical system, and in particular to a multi-beam scanning optical system that scans a plurality of beams across an object surface to form an image thereon.
In a multi-beam scanning optical system, multiple beams are deflected simultaneously by a single reflection surface of a deflector such as a polygon mirror. The deflected beams simultaneously scan across an object surface to form an image thereon. Thus, the multi-beam scanning optical system is capable of fast printing.
Various kinds of light sources are utilized in the multi-beam scanning optical system. Examples of such light sources includes a single element having a plurality of light emitting points such as that disclosed in Japanese Patent Application Provisional Publication SHOU 57-54914. Another example of the light source is a device disclosed in Japanese Patent Application Provisional Publication SHOU 60-126620, which is composed of a plurality of light emitting elements each having a single light emitting point.
In the multi-beam scanning optical system, the light source is provided such that the light emitting points thereof are arranged in an auxiliary scanning direction (which is perpendicular to a main scanning direction in which the beam spots are scanned on the object surface). The multiple light beams emitted from the light source are converged on the object surface by the scanning optical system and scanned thereacross to form a plurality of scanning lines. Since there is a finite spacing between each pair of adjacent light emitting points irrespective of the type of light source, there is also a finite spacing between each pair of adjacent scanning lines formed on the object surface. If a high quality, or high resolution, printing is desired, this spacing between adjacent scanning lines should be made small or the adjacent scanning lines should be even overlapped on each other.
Conventionally, various methods for eliminating the spacing between adjacent scanning lines to make them overlap one another are developed.
In one exemplary method, an aperture is located at a pupil position of a line image forming lens which converges each beam emitted from the light source into a line image in a vicinity of a polygon mirror. The aperture size is reduced while keeping the light emitting points arranged in the auxiliary scanning direction. As a result, the sizes of the beam spots formed on the object surface increase and the scanning lines formed by these enlarged beam spots become to overlap one another.
In this method, however, a large part of the energy of each beam is cut off by the aperture. Thus, the energy efficiency of the scanning optical system is very low.
In another exemplary method, the light emitting points of the light source are arranged in a slanting direction against the main scanning direction to reduce the distance in the auxiliary scanning direction of the beam spots formed on the object surface.
If a multi-beam semiconductor laser is utilized as the laser source, the major diameter direction of the far field pattern of each laser beam inclines against the auxiliary scanning direction. This inclination of the major diameter direction reduces the overlap between the adjacent scanning lines. Thus, an anamorphic lens or a slit is disposed on the path of the laser beams to adjust the major diameter direction of each laser beam to the auxiliary scanning direction. The addition of the anamorphic lens or the slit is not desirable since the costly anamorphic lens increases the total cost of the scanning optical system, and the slit reduces the energy efficiency of the scanning optical system.
Therefore, there is a need for a multi-beam scanning optical system that forms a plurality of scanning lines on an object surface, in which adjacent scanning lines overlap one another, without utilizing costly anamorphic lenses and without significantly reducing the energy efficiency.